Laminated quarter-wave plate or circularly polarizing plate, liquid-crystal display device using the same and method for producing the same

ABSTRACT

A laminated quarter-wave plate having: a laminate of a quarter-wave plate and a half-wave plate, wherein: the quarter-wave plate and the half-wave plate are laminated on each other so that directions of in-plane slow axes intersect each other; and each of the quarter-wave plate and the half-wave plate satisfies a relation Nz=(nx−nz)/(nx−ny)&gt;1.05 in which nx and ny are in-plane main refractive indices, and nz is a thicknesswise refractive index; and a circularly polarizing plate having: a laminate of a laminated quarter-wave plate defined above and a polarizer.

[0001] The present application is based on Japanese Patent ApplicationNo. 2001-307749, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a laminated quarter-wave plateor a circularly polarizing plate adapted to compensation forbirefringence, a liquid-crystal display device using the same and amethod for producing the same.

[0004] 2. Description of the Related Art

[0005] As a wave plate capable of providing a retardation of a quarterwavelength in a wide wavelength range of visible light, there is known alaminated quarter-wave plate in which a quarter-wave plate and ahalf-wave plate produced by uniaxial stretching are laminated on eachother so that directions of in-plane slow axes of these plates intersecteach other. For example, the laminated quarter-wave plate is widely usedfor the purpose of anti-reflection of a liquid-crystal display device(hereinafter referred to as “LCD”).

[0006] A TFT (Thin Film Transfer) drive type twisted nematic (TN) LCD iswidely used in a notebook type personal computer, a monitor, etc. TheTN-LCD has a disadvantage in that the viewing angle thereof is narrow.AVA- or IPS-LCD capable of providing a wide viewing angle have beendeveloped and begun to be popularized for monitor use. The VA-orISP-LCD, however, needs backlight electric power because the VA- orISP-LCD is lower in luminance than the TN-LCD. Moreover, the VA- orISP-LCD has been not applied to a notebook type personal computerrequiring low electric power consumption.

[0007] On the other hand, it is known that frontal luminance of amulti-domain VA-LCD is improved when the laminated quarter-wave plate isdisposed in one interlayer between a liquid-crystal cell and a polarizerwhile a laminated quarter-wave plate capable of providing circularlypolarized light of reverse rotation is disposed in another interlayer.In this method, it is however difficult to obtain a wide viewing angle.This problem can be solved by the related art when a negative uniaxialphase retarder produced by biaxial stretching and a polarizer arelaminated on each other. There is, however, a disadvantage in that theresulting film is made thick because a large number of plates must belaminated as well as production efficiency is made low because a largenumber of laminating steps is required.

SUMMARY OF THE INVENTION

[0008] To solve the problems in the related art, on object of theinvention is to provide a quarter-wave plate or a circularly polarizingplate adapted to compensation for birefringence, a liquid-crystaldisplay device using the same and a method for producing the same.

[0009] In order to solve the object, according to the invention, thereis provided a laminated quarter-wave plate having: a laminate of aquarter-wave plate and a half-wave plate, wherein: the quarter-waveplate and the half-wave plate are laminated on each other so thatdirections of in-plane slow axes intersect each other; and each of thequarter-wave plate and the half-wave plate satisfies a relationNz=(nx−nz)/(nx−ny)>1.05 in which nx and ny are in-plane main refractiveindices, and nz is a thicknesswise refractive index. Further, accordingto the invention, there is provided a circularly polarizing platehaving: a laminate of a laminated quarter-wave plate defined above and apolarizer.

[0010] Further, according to the invention, there is provided a pressuresensitive adhesive agent-including laminated quarter-wave plate having:a laminated quarter-wave plate defined above; and a pressure sensitiveadhesive layer provided on at least one of opposite surfaces of thelaminated quarter-wave plate.

[0011] Further, according to the invention, there is provided a pressuresensitive adhesive agent-including circularly polarizing plate having: acircularly polarizing plate defined above; and a pressure sensitiveadhesive layer provided on at least one of opposite surfaces of thecircularly polarizing plate.

[0012] Further, according to the invention, there is provided aliquid-crystal display device having: a liquid-crystal cell; and alaminated quarter-wave plate or circularly polarizing plate definedabove, or a pressure sensitive adhesive agent-including laminatedquarter-wave plate or circularly polarizing plate defined above anddisposed on at least one of opposite surfaces of the liquid-crystalcell.

[0013] Next, according to the invention, there is provided a method ofproducing a phase retarder (such as a quarter-wave plate or a half-waveplate) satisfying a relation Nz=(nx−nz)/(nx−ny)>1.05 in which nx and nyare in-plane main refractive indices, and nz is a thicknesswiserefractive index, the method having the step of: stretching atransparent polymer film with a thickness of 5 to 500 μm by one oftenter lateral stretching and biaxial stretching.

[0014] Features and advantages of the invention will be evident from thefollowing detailed description of the preferred embodiments described inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In the attached drawings:

[0016]FIG. 1 is a sectional view of an embodiment of a liquid-crystaldisplay device; and

[0017]FIG. 2 is a sectional view of another embodiment of aliquid-crystal display device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] As shown in FIGS. 1 and 2, the laminated quarter-wave plate 1according to the invention has a laminate of a quarter-wave plate 2 anda half-wave plate 3. The quarter-wave plate 2 and the half-wave plate 3are laminated on each other so that directions of in-plane slow axesintersect each other, and each of the quarter-wave plate 2 and thehalf-wave plate 3 satisfies a relation Nz=(nx−nz)/(nx−ny)>1.05 in whichnx and ny are in-plane main refractive indices, and nz is athicknesswise refractive index. nx is an in-plane refractive index inthe direction in which the in-plane refractive index becomes maximumwithin the plane of the plate, and ny is an in-plane refractive index inthe direction orthogonal to the direction of nx.

[0019] Materials of the quarter-wave plate and the half-wave plate arenot particularly limited but materials excellent in birefringencecontrollability, transparency and heat resistance may be preferablyused. Polymer films produced by an extrusion or cast film-forming methodare preferably used from the point of view of reducing variation inbirefringence. Examples of polymer for forming such polymer filmsinclude polyolefin (polyethylene, polypropylene, etc.),polynorbornene-based polymer, polyvinyl chloride, polystyrene,polyacrylonitrile, polycarbonate, polyester, polysulfone, polyallylate,polyvinyl alcohol, polymethacrylate ester, polyacrylate ester, celluloseester, and soon. Particularly, polynorbornene-based polymer,polycarbonate, polyester, polysulfone and polyallylate are preferredfrom the point of view of birefringence controllability, birefringenceuniformity, transparency and heat resistance. As each of the polymerfilms, a film having a thickness of not larger than 3 mm, particularlyin a range of from 1 μm to 1 mm, more particularly in a range of from 5to 500 μm is generally used from the point of view of obtaining ahomogeneous stretched film by a stable stretching process.

[0020] The method of generating a retardation is not particularlylimited. A general stretching process such as uniaxial stretching orbiaxial stretching can be used as the method. To obtain a quarter-waveplate and a half-wave plate satisfying the relationNz=(nx−nz)/(nx−ny)>1.05, tenter lateral stretching or biaxial stretchingis preferably used. The biaxial stretching may be either simultaneousbiaxial stretching using a full tentering method or successive biaxialstretching using a roll tentering method. Stretching conditions such asstretching temperature, stretching rate, stretching magnification, etc.are not determined because optimal conditions vary in accordance withthe kind of the polymer film used, the thickness thereof, and so on. Itis however preferable that the stretching temperature is near to or notlower than the glass transition point (Tg) of the polymer film used. Thestretching magnification varies in accordance with the stretchingmethod, etc. but it is preferable that the polymer film is stretchedlaterally by a stretching magnification of 50 to 200% or biaxiallystretched by a stretching magnification of 50 to 200% in a mainstretching direction to thereby form a quarter-wave plate or a half-waveplate. Incidentally, the “stretching magnification of 100%” means astate in which a non-stretched film is stretched to twice.

[0021] The thickness of the quarter-wave plate or the half-wave plate isnot particularly limited and can be determined suitably in accordancewith the purpose of use. Generally, the thickness is set to be notlarger than 1 mm, preferably in a range of from 1 to 500 μm, morepreferably in a range of from 5 to 300 μm. The quarter-wave plate andthe half-wave plate produced in this manner are laminated on each otherso that the in-plane slow axes of the two plates intersect each other.Thus, a laminated quarter-wave plate is obtained. The laminating methodis not particularly limited. Any suitable material such as an adhesiveagent, or a pressure sensitive adhesive agent can be used if thematerial is high in transparency.

[0022] The material of a polarizer 4 in FIG. 2 is not particularlylimited. Any known material in the related art can be used as thematerial of the polarizer. Generally, the polarizer is provided as aplate having a polarization film and a transparent protective film as aprotective layer bonded to one or each of opposite surfaces of thepolarization film through a suitable adhesive layer.

[0023] The material of the polarization film is not particularlylimited. Examples of the material of the polarization film include: apolarization film obtained by stretching a hydrophilic polymer film suchas a polyvinyl alcohol (PVA)-based film, a partially formalizedpolyvinyl alcohol-based film or an ethylene-vinyl acetatecopolymer-based partially saponified film after adsorbing iodine and/ordichromatic dye onto the hydrophilic polymer film; and a polarizationfilm formed from a polyene-oriented film such as a dehydrate ofpolyvinyl alcohol or dehydrochlorinate of polyvinyl chloride.Especially, a polyvinyl alcohol-based film containing iodine ordichromatic dye adsorbed thereon and oriented is preferred. Thethickness of the polarization film is not particularly limited but isgenerally preferably set to be in a range of from 1 to 80 μm,particularly in a range of from 2 to 40 μm.

[0024] A suitable transparent film can be used as the material of theprotective film provided as a transparent protective layer provided onone or each of opposite surfaces of the polarization film. Especially, afilm of a polymer excellent in transparency, mechanical strength,thermal stability and moisture sealability is preferably used. Examplesof the polymer include: an acetate-based resin such as triacetylcellulose; and a polymer selected from the materials listed above in thedescription of the quarter-wave plate and the half-wave plate. Thepolymer is, however, not limited thereto. The protective layer maycontain fine particles so that a surface of the protective layer isformed to have a finely roughened structure.

[0025] The transparent protective film which can be particularlypreferably used from the point of view of polarizing characteristic,durability, etc. is a triacetyl cellulose film having a surfacesaponified with alkali. The thickness of the transparent protective filmis optional. Generally, the thickness of the transparent protective filmis set to be not larger than 500 μm, preferably in a range of from 5 to300 μm, more preferably in a range of from 5 to 150 μm for the purposeof reducing the thickness and size of the polarizer. Incidentally, whentransparent protective films are provided on opposite surfaces of thepolarization film, transparent protective films of polymers differentbetween the front and the rear may be used.

[0026] The process of bonding the polarization film and the transparentprotective film as a protective layer to each other is not particularlylimited. For example, the bonding process can be performed through anadhesive agent of an acryl-based polymer or a vinyl alcohol-basedpolymer or through an adhesive agent at least containing an aqueouscrosslinking agent for the vinyl alcohol-based polymer such as boricacid, borax, glutaraldehyde, melamine or oxalic acid. By such anadhesive agent, the transparent protective film can be prevented frombeing peeled because of the influence of humidity and heat, so that thetransparent protective film can be formed as a film excellent in lighttransmittance and the degree of polarization. Such an adhesive layer isformed as a layer obtained by applying an aqueous solution and dryingthe aqueous solution in accordance with necessity. When the aqueoussolution is prepared, another additive and a catalyst such as acid maybe mixed with the aqueous solution. Particularly an adhesive agent madefrom polyvinyl alcohol is preferably used because the adhesive agent isexcellent in adhesion to a PVA film.

[0027] The method of laminating the polarizer 4 and the laminatedquarter-wave plate 1 to produce a circularly polarizing plate 5 in FIG.2 is not particularly limited. A suitable material such as an adhesiveagent or a pressure sensitive adhesive agent can be used if the materialis high in transparency. The quarter- or half-wave plate may be used asa layer for protecting a polarizer so that the other, half- orquarter-wave plate can be bonded and laminated on the polarizer throughan adhesive agent or a pressure sensitive adhesive agent. Then, only anordinary protective film is bonded to the other side on which the half-or quarter-wave plate is not bonded. In the case of such a method, awave plate satisfying the relation Nz=(nx−nz)/(nx−ny)>1.05 is used asthe protective film provided on one side of the polarizer.

[0028] The adhesive agent (pressure sensitive adhesive agent) used forlamination of the quarter-wave plate and the half-wave plate orlamination of the laminated quarter-wave plate and the polarizer is notparticularly limited. An adhesive agent not requiring anyhigh-temperature process for curing or drying or an adhesive agent notrequiring any long-term curing or drying process is preferably used fromthe point of view of preventing the optical characteristic of thepolarizer from changing. Further, an adhesive agent prevented frompeeling under heating and humidifying conditions is preferably used. Forexample, a transparent pressure-sensitive adhesive agent such as anacryl-based adhesive agent, silicone-based adhesive agent, apolyester-based adhesive agent, a polyurethane-based adhesive agent, apolyether-based adhesive agent or a rubber-based adhesive agent may beused.

[0029] A pressure sensitive adhesive layer 6 may be provided on thelaminated quarter-wave plate 1 or the circularly polarizing plate 5 sothat the laminated quarter-wave plate 1 or the circularly polarizingplate 5 can be bonded to another member such as a liquid-crystal cell 7.The pressure sensitive adhesive layer can be formed from a suitablepressure sensitive adhesive agent according to the related art such asthe acryl-based trackifier. When the pressure sensitive adhesive layer 6provided on the laminated quarter-wave plate 1 or the circularlypolarizing plate 5 is exposed to the surface, the pressure sensitiveadhesive layer 6 may be preferably covered with a separator for thepurpose of preventing contamination until the pressure sensitiveadhesive layer 6 is put into practical use. The separator can be formedfrom a suitable leaf body of a material selected from materials listedin the description of the transparent protective film. If necessary, arelease coat made from a suitable releasant such as a silicone-basedreleasant, a long-chain alkyl-based releasant, a fluorine-basedreleasant or a molybdenum sulfide releasant may be provided on thesuitable leaf body.

[0030] Incidentally, each of layers such as the laminated quarter-waveplate, the polarization film, the transparent protective layer, thepressure sensitive adhesive layer, etc., may be treated with anultraviolet absorbent such as a salicylic ester-based compound, abenzophenol-based compound, a benzotriazole-based compound, acyanoacrylate-based compound or a nickel complex salt-based compound sothat the layer has ultraviolet absorptive power.

[0031] The laminated quarter-wave plate or the circularly polarizingplate according to the invention is effectively used as a quarter-waveplate or a circularly polarizing plate in a multi-domain VA-LCD. Inpractical use, the laminated quarter-wave plate or the circularlypolarizing plate may be formed successively and individually bylamination in the process for production of each display device.Alternatively, the laminated quarter-wave plate or the circularlypolarizing plate may be laminated in advance. In this case, there is anadvantage in that both quality stability and laminating workability areso excellent that efficiency in production of each display device can beimproved.

[0032] The laminated quarter-wave plate or the circularly polarizingplate according to the invention can be used for forming various kindsof liquid-crystal display devices 8. Particularly light leakage causedby birefringence of a liquid-crystal cell, light leakage generated incross-Nicol polarizers and color shifting which have heretofore occurredin a VA mode liquid-crystal display device can be reduced. Thus, aliquid-crystal display device having a wide viewing angle in allazimuths can be obtained. Incidentally, when the laminated quarter-waveplate or the circularly polarizing plate according to the invention ismounted on a liquid-crystal cell, it is necessary to perform design inconsideration of birefringence based on orientation of liquid crystaland it is therefore necessary to suitably adjust the retardation valueof the wave plate and the angle of intersection between the wave plateand the polarizer.

[0033] Further, for formation of the liquid-crystal display device, atleast one layer of a suitable component such as a prism array sheet, alens array sheet, a light-diffusing plate or a backlight unit may bedisposed in a suitable position.

[0034] The invention will be described below more specifically inconnection with Examples 1 and 2 and Comparative Examples 1 to 3.

EXAMPLE 1

[0035] A 100 μm-thick norbornene film (trade name “Arton Film” made byJSR Corporation) was stretched widthwise by 130% by a tenter at atemperature of 175° C. to thereby obtain a 44 μm-thick quarter-waveplate. On the other hand, the same 100 μm-thick norbornene film asdescribed above was successively biaxially stretched lengthwise by 90%and widthwise by 5% by a roll tentering system at a temperature of 175°C. to thereby obtain a 70 μm-thick half-wave plate. Then, thequarter-wave plate and the half-wave plate were laminated on each otherso that the slow axis of the quarter-wave plate makes 20° in acounterclockwise direction whereas the slow axis of the half-wave platemakes 67.5° in a clockwise direction. Thus, a laminated quarter-waveplate was obtained. Finally, linearly polarizing plates (trade name“NRF” made by Nitto Denko Corporation) were laminated on the laminatedphase retarder so that the absorption axes of the linearly polarizingplates make 0° and 90° respectively. Thus, a right-hand circularlypolarizing plate and a left-hand circularly polarizing plate wereobtained.

COMPARATIVE EXAMPLE 1

[0036] A 100 μm-thick norbornene film the same as used in Example 1 wasstretched lengthwise by 25% at a temperature of 175° C. to therebyobtain a 91 μm-thick quarter-wave plate. On the other hand, the same 100μm-thick norbornene film as described above was stretched lengthwise by80% at a temperature of 175° C. to thereby obtain a 78 μm-thickhalf-wave plate. Then, the quarter-wave plate and the half-wave platewere laminated on each other so that the slow axis of the quarter-waveplate makes 20° in a counterclockwise direction whereas the slow axis ofthe half-wave plate makes 67.5° in a clockwise direction. Thus, alaminated quarter-wave plate was obtained. Finally, linearly polarizingplates the same as used in Example 1 were laminated on the laminatedphase retarder so that the absorption axes of the linearly polarizingplates make 0° and 90° respectively. Thus, a right-hand circularlypolarizing plate and a left-hand circularly polarizing plate wereobtained.

[0037] Values of Nz in the quarter-wave plates and the half-wave platesobtained in Example 1 and Comparative Example 1 were measured withKOBRA-21ADH which was made by Oji Scientific Instruments and which useda parallel nicol rotary method as a principle. Results of themeasurement were as shown in Table 1. TABLE 1 Nz (quarter-wave Nz(half-wave plate) plate) Example 1 1.50 1.15 Comparative Example 1 1.011.02

EXAMPLE 2

[0038] The right-hand circularly polarizing plate and the left-handcircularly polarizing plate obtained in Example 1 were disposed onopposite sides of a multi-domain VA type cell to thereby obtain aliquid-crystal display device.

COMPARATIVE EXAMPLE 2

[0039] The right-hand circularly polarizing plate and the left-handcircularly polarizing plate obtained in Comparative Example 1 weredisposed on opposite sides of a multi-domain VA type cell to therebyobtain a liquid-crystal display device.

COMPARATIVE EXAMPLE 3

[0040] Linearly polarizing plates (the same as used in Example 1) weredisposed on opposite sides of a multi-domain VA type cell to therebyobtain a liquid-crystal display device.

[0041] In each of the liquid-crystal display devices obtained in Example2 and Comparative Examples 2 and 3, frontal luminance and viewing anglesin Co≧10 in accordance with azimuth angles Φ of 45°, 135°, 225° and 315°were measured with EZ-contrast (made by Eldim). Incidentally, luminancewas normalized on the assumption that luminance in Comparative Example 3was regarded as 100. Results of the measurement were as shown in Table2. TABLE 2 Frontal Viewing angle Luminance Φ = 45° 135° 225° 315°Example 2 130 60 65 60 65 Comparative 130 40 40 40 40 Example 2Comparative 100 30 30 30 30 Example 3

[0042] It is obvious from results in Table 2 that frontal luminance isimproved and a liquid-crystal display device with a wide viewing angleis obtained when circularly polarizing plates using laminatedquarter-wave plates according to the invention are used in theliquid-crystal display device.

[0043] As described above, the laminated quarter-wave plate and thecircularly polarizing plate according to the invention can be producedeasily and inexpensively, so that productivity is excellent. When thelaminated quarter-wave plate or the circularly polarizing plate ismounted on a liquid-crystal display device, frontal luminance can beimproved so that the liquid-crystal display device can be achieved as aliquid-crystal display device with a wide viewing angle. Moreover, whenthe laminated quarter-wave plate or the circularly polarizing plate ismounted on a VA mode liquid-crystal display device, the liquid-crystaldisplay device can be provided as a VA-LCD excellent in visibility.Accordingly, the invention is of great industrial value.

[0044] This invention should not be limited to the embodiments describedabove. Various modifications can be included in this invention within arange which can be easily realized by those skilled in the art withoutdeparting from the spirit of the scope of claim.

What is claimed is
 1. A laminated quarter-wave plate comprising: aquarter-wave plate; and a half-wave plate laminated on saidquarter-plate, so that directions of in-plane slow axes of said platesintersect each other; wherein each of said quarter-wave plate and saidhalf-wave plate satisfies a relation Nz=(nx−nz)/(nx−ny)>1.05 in which nxand ny are in-plane main refractive indices, and nz is a thicknesswiserefractive index.
 2. A circularly polarizing plate comprising: alaminated quarter-wave plate according to claim 1 and a polarizerlaminated on said laminated quarter-wave plate.
 3. A laminatedquarter-wave plate according to claim 1, further comprising a pressuresensitive adhesive layer providing on at least one of opposite surfacesof said laminated quarter-wave plate.
 4. A circularly polarizing plateaccording to claim 2, further comprising a pressure sensitive adhesivelayer provided on at least one of opposite surfaces of said circularlypolarizing plate.
 5. A liquid-crystal display device comprising: aliquid-crystal cell; and a laminated quarter-wave plate according toclaim 1 and disposed on at least one of opposite surfaces of saidliquid-crystal cell.
 6. A liquid-crystal display device comprising: aliquid-crystal cell; and a laminated quarter-wave plate according toclaim 3 and disposed on at least on of opposite surfaces of saidliquid-crystal cell.
 7. A liquid-crystal display device comprising: aliquid-crystal cell; and a circularly polarizing plate according toclaim 2 and disposed on at least one of opposite surfaces of saidliquid-crystal cell.
 8. A liquid-crystal display device comprising: aliquid-crystal cell; and a circularly polarizing plate according toclaim 4 and disposed on at least one of opposite surfaces of saidliquid-crystal cell.
 9. A method of producing a phase retardersatisfying a relation Nz=(nx−nz)/(nx−ny)>1.05 in which nx and ny arein-plane main refractive indices, and nz is a thicknesswise refractiveindex, said method comprising the step of: stretching a transparentpolymer film with a thickness of 5 to 500 μm by one of tenter lateralstretching and biaxial stretching.